Ethylene/vinyl alcohol-based copolymers (hereinafter referred to as “EVOH” for short) obtained by saponifying ethylene/vinyl acetate copolymers or the like are particularly excellent in gas barrier properties against oxygen and other gases, aroma retention, oil resistance and chemical resistance and therefore are properly used in the form of various films, sheets, containers and like molded articles. They are also attracting attention as protective coating materials for metal surfaces, paper, wood and timber, among others. They are particularly excellent in gas barrier properties as compared with other resin films and therefore are very useful as food packaging materials such as films, sheets, laminates, hollow containers and the like, which are required to be capable of preventing the contents from being oxidized and/or the aroma from being lost.
As for the methods of molding EVOH into films and sheets, among others, the method comprising melt extruding or injection molding EVOH and the method comprising laminating EVOH films are generally and widely adopted. On the other hand, the technique of applying solutions or aqueous dispersions of EVOH, followed by drying has been proposed since they can form relatively thin coatings and can readily form coatings on hollow containers and other articles complicated in shape.
However, with the increase in EVOH solid content, EVOH solutions show increases in viscosity, making it difficult to use them; further, there are such problems as working environment deterioration by organic solvent evaporation in the coatings formation process and economic disadvantage due to the necessity of an apparatus for recovering the organic solvent, since the solutions comprises a organic solvent such as dimethyl sulfoxide or a mixed solvent composed of an alcohol and water. On the contrary, the method comprising applying an aqueous EVOH dispersion is considered as advantageous, hence expected, from the working environment and economic viewpoint, since the dispersion medium is water.
As regards aqueous EVOH dispersions, proposals have been made, for example, of a method of producing emulsified dispersions of a saponified ethylene/vinyl acetate copolymer which comprises dissolving a saponified ethylene/vinyl acetate copolymer with warming in the presence of an alcohol, water and a surfactant and emulsifying and dispersing the solution while removing the alcohol under forced high-speed stirring (Patent Document 1), a method of producing aqueous emulsified dispersions of saponified ethylene/vinyl acetate copolymers which comprises maintaining an emulsified dispersion of a saponified ethylene/vinyl acetate copolymer at a pH of not higher than 5.5 to thereby cause the emulsified dispersion to aggregate, filtering off the resulting aggregate and neutralizing the filtered aggregate obtained using a basic substance with stirring for redispersion (Patent Document 2), a method of producing aqueous emulsified EVOH dispersions which comprises emulsifying and dispersing a copolymer resulting from block-wise or graft-wise bonding between an EVOH component and an ionic group-containing component and insoluble in water at ordinary temperature (Patent Document 3), an aqueous dispersion comprising an ionic group-containing EVOH species as a dispersion stabilizer and EVOH as a dispersoid (Patent Document 4), a method of producing EVOH dispersions which comprises concentrating the resinous material by centrifugation during the process for producing EVOH dispersions (Patent Document 5), and an aqueous dispersion of a specific EVOH copolymer modified with a peroxide, for instance (Patent Document 6).
However, such aqueous EVOH dispersions have problems; namely they are poor in long-term storage stability or insufficient in film-forming properties.
On the other hand, an aqueous EVOH dispersion has been proposed in which a base-neutralized ethylene/α,β-unsaturated carboxylic acid copolymer is used as a dispersion stabilizer (Patent Document 7).
The above aqueous EVOH dispersion is excellent in long-term storage stability even at a high solid matter concentration and the coatings obtained therefrom are excellent in gas barrier properties. However, when the above dispersion is subjected to low-temperature or short-time drying or applied to give thick coatings having a thickness of 10 μm or thicker, for instance, a number of cracks appear on the coat surface, sometimes leading to inferior gas barrier properties; improvements in film-forming properties are demanded.    Patent Document 1: Japanese Kokai Publication Sho-54-101844    Patent Document 2: Japanese Kokai Publication Sho-56-61430    Patent Document 3: Japanese Kokai Publication Hei-04-225008    Patent Document 4: Japanese Kokai Publication Hei-05-93009    Patent Document 5: Japanese Kokai Publication Hei-05-179001    Patent Document 6: Japanese Kokai Publication Hei-07-118471    Patent Document 7: WO 03/025058